FR3043430B1 - DEVICE FOR POST-PROCESSING EXHAUST GASES OF A THERMAL ENGINE - Google Patents

Abstract

The invention relates to a device for the aftertreatment of the exhaust gases of a combustion engine (1) which comprises, from upstream to downstream: a DOC upstream oxidation catalyst element (2) or a trap organ NOx LNT; A mouth of a means (6) for introducing a reducing agent or precursor of a reducing agent for the selective catalytic reduction of the nitrogen oxides SCR; SCR selective catalytic reduction means of the nitrogen oxides, said means comprising: a selective catalytic reduction member SCR of the nitrogen oxides NOx (3) dedicated, and / or a catalytic selective reduction coating SCRF of the oxides NOx nitrogen on a particulate filter member (4); • the particulate filter member (4); • a DOCNOx downstream oxidation catalyst organ with NOx storage function (5).

Description

DEVICE FOR POST-PROCESSING EXHAUST GASES OF A THERMAL ENGINE

[001] The invention relates to means for treating pollutants from the exhaust gases of heat engines.

[002] The pollutant emissions of combustion engines fitted to motor vehicles are regulated by standards. Regulated pollutants are, depending on the combustion engine technology considered, carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxides (NOx, ie NO and NO2) and particles (PM), which are formed during combustion of the fuel in the combustion chamber and then emitted to the exhaust.

[003] It is known to employ a number of pollution control means in the exhaust line of combustion engines to limit the emissions of regulated pollutants. An oxidation catalyst allows the treatment of carbon monoxide, unburned hydrocarbons, and under certain conditions nitrogen oxides; a particulate filter can be used for the treatment of soot particles.

[004] This type of device is generally designated by the term "after-treatment" device of the exhaust gases.

[005] To meet pollution control standards on nitrogen oxide (NOx) emissions, a specific post-treatment system can be introduced into the exhaust line of vehicles, including vehicles equipped with diesel engines. For the treatment of nitrogen oxides (NOx), selective catalytic reduction (SCR) technologies for Selective Catalytic Reduction are known in English, which consist in reducing NOx by introducing a reducing agent ( or a precursor of such a reducing agent) in the exhaust gas by catalyzed reactions. It may for example be a urea solution, the decomposition of which will make it possible to obtain ammonia which will serve as a reducing agent, but also of a reducing agent or a precursor of such a reducing agent. gaseous form. In the remainder of the present document, a "reducing agent" is generally used to denote a reducing agent or a reducing agent precursor.

The reducing agent generated makes it possible to reduce the nitrogen oxides by reaction in an SCR catalyst, that is to say a substrate carrying a catalytic impregnation capable of promoting the reduction of NOx by the reducing agent.

[007] European standards, in particular, tend to become more and more severe. And solutions to reduce pollutant emissions at the end of the exhaust line to meet current standards will prove insufficient given the changes in standards planned beyond 2017.

[008] Indeed, the first step of the standard, Euro 6b (effective in September 2014) led car manufacturers to choose between different options to specifically reduce NOx emissions: - NOx reduction "at the source >>, at the level of the actual operation of the engine, via technologies of the exhaust gas recirculation type in the engine, recycling also called EGR technology according to the acronym of the English term corresponding to "Exhaust Gas Recirculation" high and low pressure, for example; - the reduction of NOx at the exhaust line via a sequential catalytic treatment technology called "NOx trap"; the reduction of NOx at the level of the exhaust line also, via a continuous treatment technology called "selective catalytic reduction" as briefly described above (SCR); even by cumulating many of these solutions.

[009] If these solutions make it possible to satisfy this first step in the evolution of the standard (Euro6b), they are not necessarily capable of satisfying the second stage which promises to be even more severe (Euro 6c, entry into force foreseen in September 2017), with measurements of pollutants on a new rolling cycle called "WLTC" (for "Worldwide Harmonized Light Vehicle Test Cycle" in English, ie harmonized test cycle for light vehicles in French), containing more phases transient than the current approval cycle (known as "MVEG" for Motor Vehicle Emissions Group in English, which is a group of emissions for motorized vehicles in French), but also out-of-cycle measures (called "RDEs" for Real Driving Emission or emissions under real driving conditions) should be introduced.

[0010] In particular, to meet the risks of excessively high NOx emissions outside the cycle, various technological solutions and architectures can be envisaged. They have their advantages and disadvantages. But the most efficient nitrogen oxide treatment technology is selective catalytic reduction (SCR) because it is efficient in more extended temperature and gas flow ranges than a NOx trap, the other solution post-processing.

In addition, there are constraints of implantation of the post-processing device. In fact, in general, the catalytic systems used are all the more effective as the temperature of the exhaust gases passing through them is high (to a certain extent). They will then start all the more quickly after starting the engine as the temperature of the exhaust gas increases rapidly. It is therefore advantageous to implement the after-treatment devices as close to the engine, that is to say closer to the exhaust manifold, under hood, even though this environment is generally very crowded. The post-processing devices must therefore be as compact as possible without affecting their performance.

Throughout the present text, the terms "upstream" and "downstream" are understood as a function of the general direction of flow of the exhaust gases in the exhaust line integrating the post-processing units, since the engine to the end cannula of the exhaust line.

It is, for example, known from the patent application WO 2011/089330 a post-processing device grouping in the same envelope several bodies that will be successively traversed by the exhaust gas. It is proposed, in particular, a series of organs comprising upstream downstream: - an oxidation catalyst, - a urea reducing agent injector, - a mixer whose role is to intimately mix the droplets of urea injected into the envelope traversed by the gases, so as to decompose to ammonia as homogeneously as possible over the entire cross section of the envelope, - an organ SCR, - a particulate filter (called FAP by the after). It also proposes an alternative, consisting of replacing the SCR member and the FAP, with a FAP which is impregnated with a NOx reduction catalyst and which thus fulfills both the soot filter function and the reduction of the NOx (called SCRF later).

In addition, additional constraints arise when the motor vehicle is a vehicle called "heavy" (more than 1500 kg), whether a vehicle for private or utility type. In fact, under the same driving conditions as a lighter vehicle, the "heavy" vehicle will have higher temperature conditions in the exhaust to be managed, and larger quantities to process NOx generated in the engine. To compensate for these higher NOx emissions, the amounts of reducing agent to be injected into the exhaust line (for example urea decomposing to ammonia) will have to be larger too, since these quantities are dictated by the stoichiometry of the NOx reactions with ammonia. The higher temperatures of the gases at the engine outlet also favor the thermo-desorption of the ammonia stored in the SCR (and / or SCRF) members, and can furthermore contribute to the degradation of their active catalytic phases which can induce a decrease in their storage capacity in ammonia. The combination of higher temperatures and a higher amount of urea (or ammonia) to be injected on the line induces an increased risk of ammonia emissions that would not react at the end of the exhaust line. . Ammonia leaks at the end of the exhaust line are smelly, and can be inconvenient, especially if the vehicle is in a confined space of closed parking type.

[0015] It is thus known from patent FR 2 945 962 a post-treatment device comprising a SCR member using a catalyst in the form of an acid zeolite which is not or very slightly exchanged, which has a capacity to store more ammonia at low temperature than other zeolite type zeolites exchanged. However, it does not guarantee to completely avoid the escape of ammonia at the exit of the exhaust line, in particular by desorption of ammonia at high temperature.

The invention then aims to design a post-treatment of the engine exhaust gas that overcomes the aforementioned drawbacks. One of its aims is to improve existing systems to meet higher standards for pollutant emissions, and more particularly for NOx emissions under unstabilized rolling conditions of the urban rolling type and / or an extended temperature range, while limiting as much as possible leakage of reducing agent NOx unreacted end of the exhaust line. Advantageously, it also aims to obtain a postprocessing device that is more efficient and which remains, in addition, compact.

The invention firstly relates to a device for post-treatment of the exhaust gas of a combustion engine which comprises, upstream to downstream:> a DOC oxidation upstream oxidation member or a NOx trap organ LNT; a mouth of a reducing agent introduction means or a precursor of a reducing agent for the selective catalytic reduction of the SCR nitrogen oxides; means for selective catalytic reduction SCR of nitrogen oxides, said means comprising: a selective catalytic reduction member SCR of nitrogen oxides NOx dedicated, and / or a catalytic selective reduction coating SCRF of nitrogen oxides NOx on a particulate filter element; > the particulate filter element; > a downstream oxidation catalyst member with NOx storage function.

According to one embodiment of the invention, the upstream oxidation catalyst member comprises a PNA nitrogen oxide adsorber material.

Preferably, the downstream oxidation function storage organ NOx comprises a metal or oxidizing metals (s) selected from Pt and / or Pd and NOx storage materials in the form of nitrates and or nitrites.

[0020] Advantageously, the downstream oxidation-catalyzing organ with NOx storage function according to the invention comprises a NOx storage material (s) in the form of nitrates and / or nitrites from elements with basic properties, especially in the form of oxide (s) or aluminate (s) of metals such as barium Ba, neodymium Nd, lanthanum La or praseodymium Pr, or in the form of mixed metal oxides such as solid solutions of Ce-Zr-Pr, Zr-Ce-La, Zr-Pr, Ce-Zr-La-Pr, Al-Ba, Al-Ba-Ce or Al-La-Ce.

The downstream oxidation catalyst member NOx storage function of the invention is particularly free of rhodium.

According to one embodiment, the NOx storage function of the oxidation downstream of the invention is in the form of a matrix or "honeycomb" on the walls of which is deposited a catalytic coating in the form of a layer or a stack of layers.

A preferred embodiment of the invention consists in that the selective catalytic reduction means SCR nitrogen oxides, include: - a selective catalytic reduction member SCR NOx nitrogen oxides dedicated, and - a selective catalytic reduction coating SCRF NOx nitrogen oxides on a particulate filter member, said catalyst member for selective catalytic reduction of nitrogen oxides preferably being of a length at least two times smaller, especially at least 2, 2 to 3 times smaller than the length of the particulate filter.

Preferably, the catalytic selective catalytic reduction member of the nitrogen oxides has a length of at most 80 mm, in particular at most 76 mm, preferably between 45 and 55 mm.

The invention also relates to an exhaust line of a heat engine, which incorporates the post-processing device described above.

The invention also relates to a motor vehicle defining a space under the hood and a space under the body, and comprising a heat engine connected to the exhaust line according to the preceding claim, such as the engine and the post-load device. treatment of the exhaust line are arranged in the space under hood.

Alternatively, the invention also relates to a motor vehicle delimiting a space under the hood and a space under the body, and comprising a heat engine connected to the exhaust line described above, such as: - the engine, the DOC upstream oxidation catalyst member or a NOx LNT trap member, the mouth of a reducing agent introduction means or a reducing precursor means for the selective catalytic reduction of SCR nitrogen oxides, the means for SCR selective catalytic reduction of the nitrogen oxides and the particulate filter member are arranged in the undercap space, and the downstream oxidation-catalyzing element with NOx storage function is arranged in the underbody space.

The invention also relates to a method of implementation of the post-processing device described above, such as: - it under-injects the reducer or the precursor gearbox, by the mouth of the introduction means of a reducing agent or precursor of a reducing agent for the selective catalytic reduction of the nitrogen oxides SCR, so as to saturate in NOx the downstream oxidation-catalyzing mechanism with NOx storage function, and then the reducer is over-injected; or the reducing precursor for converting the maximum NOx by reduction, the excess NH3 at the outlet of the particulate filter then being oxidatively treated by the downstream oxidation-functional NOx storage catalyst as long as said organ contains NOx stored.

The invention therefore proposes the addition of an oxidation catalyst capable of storing NOx at the end of the architecture of the post-treatment device. It turned out that this type of catalytic material ,, could very effectively treat possible residual ammonia leakage, depending on the reaction mechanisms that will be detailed later. Very advantageously, this type of material is simple to produce, simple to integrate into the exhaust line and simple to implement to participate in the treatment of exhaust gas. The invention has thus found a new use and a new location for oxidation catalyst materials, generally arranged upstream of the after-treatment devices.

The post-treatment device may also comprise a NOx sensor between the particulate filter member provided with a selective catalytic reduction catalyst coating SCRF NOx nitrogen oxides or the SCR member and the oxidation catalyst downstream, and possibly another NOx sensor upstream of the DOC upstream oxidation catalyst member / LNT NOx trap, and / or another NOx sensor downstream of the downstream oxidation catalyst. The "upstream" sensor, upstream of the DOC upstream catalyst or the NOx trap, can also be replaced by modeling if necessary.

Preferably, the upstream oxidation catalyst member has a catalyst whose amount of noble metals is adjusted so as to obtain at the outlet of the organ exhaust gas whose NO2 / NOx ratio is equal or similar of 0.5 (it is understood by "neighbor" a variation of for example +/- 15% around this value).

Preferably, the catalyst of the particulate filter, when it is provided, is based on zeolite (s) exchanged (s) copper.

It should be noted that the copper-exchanged zeolites proposed for the SCRF and / or exchanged with iron for the catalyst of the catalytic reduction catalyst member SCR are for example based on zeolites of the chabazite, ferrierite or hydrated aluminosilicate type (ZMS5). ), and may also contain at least one of the following oxides: cerium (Ce) oxide, zirconium (Zr), or at least one of the following metals: niobium (Nb), tungsten (W), titanium ( Ti).

Advantageously, the after-treatment device according to the invention also comprises a mixing member of the exhaust gas and the reducing agent and / or the precursor of the gearbox between the mouth of the gearbox introduction means and / or precursor of a reducing agent for the selective catalytic reduction of SCR nitrogen oxides and the selective catalytic reduction catalytic converter of nitrogen oxides integrated in the SCRF particulate filter.

This mixer has the function of mixing as well as possible the exhaust gas with the reducing agent or reducing precursor, this being particularly useful when the precursor is of the liquid type, such as urea in aqueous phase.

The invention also applies to the direct injection of the reducing gas, such as ammonia, which feeds the exhaust line from one or more salt cartridges (including SrCI2 type) suitable adsorbing the ammonia and releasing it by thermal activation, in a known manner (commonly called "solid" SCR technology), and in this case, the mixer is less necessary.

The invention also relates, in a first embodiment, to a motor vehicle defining a space under the hood, which contains what is usually referred to as the engine compartment, and a space under the box, and comprising a a heat engine connected to the preceding exhaust line, such that the engine and the aftertreatment device of the exhaust line are arranged in the space under the hood. In this way, all the depollution units are grouped compactly close to the engine.

The invention also relates, in a second embodiment, to a motor vehicle defining a space under the hood and a space under the body, and comprising a heat engine connected to the previous exhaust line, such as the engine. the DOC upstream oxidation catalyst member or the LNT NOx trap, the mouthpiece, and the particulate filter member provided with a catalytic selective catalytic reduction SCRF coating of NOx nitrogen oxides of the post-injection device. treatment of the exhaust line are arranged in the space under the hood, and such that the downstream oxidation catalyst is disposed in the underbody space. This brings together, as close as possible to the engine, the abatement members needing a high exhaust gas temperature and the engine is removed from the downstream oxidation catalyst treating the ammonia leaks in order to preserve it from too severe thermals which could cause a degradation of its effectiveness.

The invention also relates to a method of implementation of a motor vehicle comprising a heat engine connected to the exhaust line as described above, the treatment of NOx being provided by two systems, d ' on the one hand the NOx trapping member when it is used, and on the other hand the SCR and / or SCRF assembly, and, depending on the availability of the reducing agent on the vehicle, adjusting the NOx treatment weighting between the one or other of the systems. With the invention, the treatment of NOx can be further supplemented by the downstream oxidation catalyst to treat ammonia, since it is also able to store NOx.

The invention is described in more detail below with reference to the figures relating to a non-limiting example of embodiment relating to a device for aftertreatment of the exhaust gas of a diesel engine: - Figure 1 schematically represents a motor and its exhaust line of a motor vehicle comprising a post-processing device according to an example of the invention; - Figures 2 and 3 show the after-treatment device according to the invention according to two embodiments; FIG. 4 is a graph showing the leakage of residual NH 3 as a function of the temperature during operation under normal conditions of the aftertreatment device of the example according to the invention; FIG. 5 is a graph making it possible to visualize the temperatures at the outlet of some of the members constituting the post-processing device of the invention in a given rolling cycle.

The references taken from one figure to another designate the same components, and the various components shown are not necessarily scaled. The figures remain very schematic to facilitate reading.

FIG. 1 represents the post-processing device of the invention integrated in the exhaust line 10 of a heat engine 1. This device comprises a series of members that can be assembled in different configurations, either in a single common envelope connected to the line, or in several envelopes connected to the line and in which are distributed the different organs. The general flow of exhaust gas flow from the engine 1 to the end of the exhaust line 10 is symbolized by the arrow F, and it is with reference to this direction of flow that the the terms "upstream" and "downstream" are used.

According to a first example, this post-treatment device comprises, from upstream to downstream, a first member 2 upstream oxidation catalyst (DOC), a member SCR 3, a particulate filter (FAP) 4 possibly provided with of a catalytic coating SCR (usually referred to as SCRF), and a DOCnox downstream oxidation catalyst member with NOx storage function 5. It also comprises an injector 6 of ammonia precursor (in the form of urea in aqueous phase) capable of injecting the precursor on the exhaust line 10 upstream of the SCR member 3 and, in known manner, in fluid connection with a precursor tank 7. It also comprises a not shown mixer (also called mixing box), already known, in particular from the patent application WO 2011/089330, which will ensure a sufficiently intimate mixture between the drops of urea and the exhaust gases to facilitate the decomposition of urea.

The oxidation catalyst 2 may be replaced / supplemented functionally by a NOx trap, for example consisting of a cordierite type honeycomb support on which is deposited a catalytic phase comprising storage promoting elements such as that, but not only, simple or mixed oxides of barium and / or magnesium.

The piloting of the injection is done by the command control unit 8 (of the on-board computer type), in particular with the aid of the information collected and reassembled at the control unit by the NOx sensors 9, 9 'and 9 ". It should be noted that at least one of these NOx sensors, and in particular the first one, 9, can be eliminated and replaced by means for estimating the NOx content in the exhaust gas stream, in particular using mappings. pre-established engine.

The control unit 8 also uses the pressure information collected and reassembled by pressure sensors 11, 11 'disposed on either side of the particulate filter 4, in particular for controlling the regeneration of the particulate filter. . It also uses the temperature information returned by the thermocouple 12 disposed between the DOC 2 and the SCR member 3.

If we detail the different bodies now, the first "brick" of this post-treatment device is the oxidation catalyst 2, which oxidizes the reducing species that are carbon monoxide (CO) and hydrocarbons unburned (HC).

It consists of a honeycomb cordierite type support on which is deposited a catalytic active phase ("washcoat") containing precious metals to catalyze oxidation reactions of CO, HC and NO. This phase also comprises oxides such as alumina doped with various stabilizers (lanthanum, cerium, zirconium, titanium, silicon, etc.). On these oxides, precious metals (Pt, Pd, Rh, Ru, Re, Au) are deposited in order to catalyze the oxidation reactions at low temperatures. Acidic compounds such as zeolites are also added. Their ability to store hydrocarbons at low temperatures and remove them from storage at high temperatures can improve the treatment of HC during cold phases. To these functions can be added (oxidation of carbon monoxide and unburned hydrocarbons and storage of these at low temperature) a storage function of nitrogen oxides, NOx also at low temperature. This storage function is ensured by the introduction of materials of simple or mixed oxides type basic type such as, for example, cerium oxides or barium among others.

The SCR catalyst of the dedicated SCR member 3 and / or the particulate filter 4 is based on copper zeolites, such as chabazite, β, copper-ferrierite, ZSM5 ... This is the best choice, in particular the SCRF 4, so that the catalyst of the particulate filter remains effective even at very high temperature (that it resists filter regenerations in particular). The porous support of the filter 4 is rather of SiC silicon carbide.

The SCRF particle filter 4 also processes the nitrogen oxides and is also provided with a SCR coating, in addition to the dedicated SCR member 3.

The invention proposes the addition of DOCnox downstream oxidation catalyst member NOx storage function 5 in the most downstream part of the aftertreatment device of the exhaust gas: this organ will allow to treat residual ammonia leakage at the outlet of the exhaust line without modifying the NOx reduction performance, as explained below: When the engine is loaded for reasons of power requirements or high speeds, for example, NOx emissions are even higher when the vehicle is heavy (especially when its weight exceeds 1500 kg). Indeed, the heavier the vehicle, the higher the temperatures reached in the engine and the higher the amount of NOx formed (Zeldovich mechanism). To compensate for these higher NOx emissions, the computer modifies the amount of urea to be injected in accordance with the stoichiometric ratio of the reaction (R2) below, which is the one that is sought to promote: (R1) standard SCR (R2) SCR with high temperature rapid kinetics (R3) SCR The consequence of this increase of NOx to be treated by the SCR member 3 and the SCR catalyst of the SCRF 4 particulate filter is an increase in the injected amount of urea. However, urea does not act directly on NOx to reduce it to nitrogen (N2) but must first decompose (see reactions R4 and R5 below) into NH3, the latter

being then stored in the active phase (also called "washcoat") of the SCR 3 organ and the SCR catalyst of the SCRF 4 (R4) particle filter. Thermolysis of the urea (R5) hydrolyzed isocyanic acid [0054 It is thus understood that there is a NH3 ammonia storage phase in the SCR 3 and SCRF 4 catalysts, prior to the conversion of NOx into N2, thanks to this same NH3 generated from urea. This ammonia storage represented by a criterion called "NH3 storage capacity" evolves according to various parameters, such as the intrinsic characteristics of the catalytic coating SCR, its state (new or aged), the intra-catalyst temperature, or even the residence time. gases in the catalyst (the latter parameter is rather second-order) ...

For a given SCR catalyst, its aging state can therefore play: the older it is, the more it tends to lose its storage capacity ammonia.

There are thus types of SCR catalyst which have more or less NH3 storage capacities: the storage capacity of the non-exchanged zeolite type SCR catalysts is generally greater than the storage capacity of the zeolite type SCR catalysts. exchanged with copper, itself generally greater than the storage capacity of zeolite SCR catalysts exchanged with iron, for example.

As shown in FIG. 4, for a given SCR catalyst having a given aging state, the ammonia storage capacity thereof changes as a function of the intra-SCR 3 or intra-SCRF 4 temperature. The figure represents a graph with the temperature in ° C in abscissa and the mass of ammonia stored by the SCR catalyst in mass, expressed in mg / l of catalyst, for this example, an iron-exchanged zeolite catalyst. . On reading this graph, we observe a decrease in this storage capacity with temperature, a decrease due to the phenomenon of thermo-desorption of ammonia.

It should also be noted that the oxidation of NH 3 to NOx occurs from 450 to 500 ° C.

It is thus understood that there are life situations where emissions of NH3 to the cannula, that is to say at the downstream end of the exhaust line, may appear, in particular: - when the NH3 can "destock" SCR catalyst by thermodisorption without reacting with NOx

- (it is redundant with the other two situations below) - when the amounts of NH3 are too large compared to the storage capacity of the SCR catalyst - when the SCR catalyst has aged thermally and has lost its storage capacity Ammonia [0060] Exhaust aftertreatment systems must be effectively controlled throughout the life of the vehicle (this is called on-board diagnostic or "OBD" for the acronym for the corresponding English terms "On Board Diagnosis"). So there are different procedures that can put the system at fault: - by "under-injecting" urea and therefore ammonia, the system must be able to detect a lower efficiency of NOx treatment - or, on the contrary, by "over-injecting" urea and specifically creating NH3 emissions downstream from the SCR 3 / SCRF 4 system, the system must be able to detect them. In the latter case, the diagnosis of the system being carried out thanks to the sensor located downstream of the SCR 3 / SCRF 4 system, the present invention eliminates this ammonia leak downstream of the DOCNOx downstream oxidation catalytic converter with storage function. NOx 5, and therefore to the cannula of the vehicle.

The optimum efficiency in reduction of NOx by the combination SCR 3 / SCRF 4 (see reaction R2 mentioned above) involves a sufficient injection of NH3, namely a = NH3 / NOx = 1 (value depending on the stoichiometry of reactions R1, R2, R3). But to be certain of having the right amount of NH3 in the SCR 3 organ and / or the SCRF 4 filter, it may be wise to put in a slight excess of NH3, with a> 1. In this case, leaks Ammonia will appear at the exit of the SCR 3 / SCRF 4 system, emissions that must also be processed before the end of the exhaust line.

The heavier the vehicle, the more these phases at risk (injected amounts of urea very important to "respond" to the amounts of NOx produced by the engine, NH3 storage capacity reduced because of high temperatures, etc.. .) can occur and be responsible for ammonia leakage from the cannula.

One way of dealing with these residual NH3 emissions / leaks is to add a specific treatment catalyst to the downstream exhaust line of the NOx treatment systems. The invention thus added, downstream of the SCR and SCRF members, the DOCnox downstream oxidation-storage organ with NOx storage function 5 which is inspired by NOx traps (or "Lean NOx Trap" or LNT in English).

The characteristics of the NOx trap are as follows. The catalytic coating generally contains three layers, one for platinum, one for palladium and one for rhodium. It also contains storage materials for nitrogen oxides in the form of nitrates and / or nitrites. These materials use chemical elements with basic properties such as barium (barium in the form of oxides or aluminates), neodymium (Nd), lanthanum (La) or praseodymium (Pr). Mixed oxides such as solid solutions of Ce-Zr-Pr, Zr-Ce-La, Zr-Pr, Ce-Zr-La-Pr, Al-Ba, Al-Ba-Ce or Al-La-Ce can also be used. to be used.

The conventional operation of a NOx trap is as follows: - it NO to NO2 oxide through platinum and / or palladium, because nitrogen monoxide is largely in the majority (90%) in nitrogen oxides (NOx) coming out of the engine. - it stores nitrogen oxides in the form of nitrates (NO3) at the barium sites (Ba (NO3) 2) - once all the storage sites are saturated, a "regeneration" of the NOx trap is triggered to clean the catalytic surface of its nitrates and to convert them into nitrogen (N2) This "regeneration" is made possible by the increase of the wealth of the engine generating an excess of reducing species in the NOx trap (HC and CO / H2 mainly). This excess of reducing agent converts the nitrates stored in nitrogen (N2) thanks to rhodium. The NOx trap thus cleaned (or regenerated), the wealth returns to a normal level for a diesel engine namely 0.15 to 0.35, and the NOx storage phase can resume. This succession of poor modes and rich modes is to optimize according to the needs (level of NOx output engine to be treated, storage capacity of the NOx trap ...). It should be noted that the capacities to store NOx, as well as to reduce them, depend on a large number of parameters (intrinsic characteristics of the NOx trap, state of aging, thermal level in the organ, etc.).

The oxidation DOCNOx downstream oxidation organ NOx storage function of the invention is in fact partially a NOx trap, which is modified for another purpose: in the invention, the storage capacity of the NOx from a NOx trap is useful, but it is not necessary, however, to have rhodium, since the reduction of stored nitrates (to regenerate the NOx trap) will not be ensured by rich purges ( and therefore by CO / H2 and HC on rh) but by the NH3, emitted at the outlet of the SCR 3 / SCRF 4 members, according to the reactions R2 and / or R3 previously described.

The operation of the DOCnox downstream oxidation catalyst member (close to a NOx trap but without rhodium) can thus be the following: - 1) it is sub-injected with urea to reduce the available quantities of NH 3 in the organs SCR 3 / SCRF 4 and effectively limit their effectiveness for treating NOx. This operation aims to fill / saturate DOCnox 5 downstream oxidation catalytic converter organ in nitrates since a part in this case of NOx will not be treated by the SCR 3 / SCRF 4 system. The NOx sensor downstream of the organ downstream oxidation catalyst DOCnox 5 could help to better control this phase to avoid emitting NOx unnecessarily to the cannula, - 2) it is then over-injected slightly in urea (a> 1) to reach the optimum efficiency. conversion of NOx by NH3, the slight excess of NH3 leaving the SCR 3 / SCRF 4 system being then treated with the nitrates stored in the downstream DOCnox oxidation catalyst organ, - 3) once the downstream oxidation catalyst DOCnox is emptied of its nitrates consumed by the previous operation 2), it is therefore no longer able to oxidize the NH3 emissions. However, there will be no NH3 emissions to the cannula as the NH3 will then be oxidized by oxygen on the DOCnox downstream oxidation catalyst member to form NOx. As soon as the engine operating conditions allow it, repeat step 1).

This solution therefore avoids any risk of leakage of NH3 to the cannula. And in the case where a residual amount of NH3 appears at the output of the SCR 3 / SCRF 4 system, it would be immediately oxidized.

By positioning the DOCNOx downstream oxidation catalyst member "behind" the SCR 3 / SCRF 4 system, downstream of it, that is to say further down the exhaust line, one protects it from thermal aging, which is very advantageous in terms of durability. Moreover, the temperatures reached in DOCNOx downstream oxidation catalyst member 5 are moderate compared to those reached by exhaust aftertreatment systems at engine output, its storage capacity is favored. Indeed, up to 350 to 400 ° C, its capacté to store the NOx in the form of nitrates is satisfactory. And since this system also needs a certain temperature level to store NOx (the oxidation of NO to NO2 necessary for storage is from 120 to 130 ° C), a metal substrate can be used for DOCnox downstream oxidation catalyst member which is associated with heating means, for example a heating resistor. This system can thus store, during the cold start of the engine, the NOx that would not have been stored by a catalyst at the motor output or treated by the SCR member 3 not yet primed. With good thermal control of DOCnox downstream oxidation catalytic converter, the exhaust line can store NOx as soon as the engine starts, while ensuring perfect conversion at higher temperatures and avoiding NH3 leaks. to the cannula.

Figures 2 and 3 show different variants of the members 2, 3, 4 and 5, which, for convenience of representation, are all shown arranged in a single envelope. However, the member 5 may equally well be in a separate envelope, further downstream in the exhaust line, this configuration being moreover preferred.

FIG. 2 represents a first embodiment that is decomposed into two variants: in a first variant A, the member 2 is a DOC / PNA, the member 3 is a thin SCR member (length much less than that of FIG. of the particulate filter), the member 4 is the particulate filter provided with a SCR coating called SCRF, and the member 5 is the downstream oxidation catalyst capable of storing the NOx; in a second variant B, the member 2 is a NOx trap LNT.

FIG. 3 represents a second embodiment also available in two variants: in a first variant C, the member 2 is a DOC / PNA, the dedicated member SCR 3 is deleted and the SCR reduction of NOx is only done by the SCR-coated particle filter 4 SCRF, which preferably has a length greater than the particulate filter of the first embodiment (and / or a higher amount of SCR coating); the second variant D consists in replacing the DOC / PNA 2 with a NOx trap LNT 2.

FIG. 5 is a graph, with the abscissa time expressed in minutes, and the ordinate (left axis) on the one hand the temperature in ° C measured by a thermocouple, on the other hand (axis on the right). the speed of the motor vehicle equipped with the engine 1 in km / h / operation of the engine 1. The curve C1 is the temperature curve measured at the output of the member 2, the curve C2 is the curve of the temperature representative of the temperature in the member 5. It is verified with this graph that by positioning the member 5 far in the exhaust line, after the particulate filter, the temperatures experienced by this member 5 are moderate, effectively protecting it from aging premature thermal and preserving its ability to store NOx [0075] In conclusion, the catalytic oxidation material with NOx storage added according to the invention in the most downstream part of the after-treatment device has a very positive impact on theammonia leakage, as well as NOx emissions over a wide range of temperatures.

Its addition does not significantly complicate the architecture of the exhaust after-treatment device as a whole (nor its implementation).

In itself, it is a simple organ in its composition and manufacture, since inspired by the composition of a NOx trap, but by simplifying it (since it contains at least one component less , rhodium or other noble metal of equivalent function).

Claims (10)

1. Apparatus for post-treatment of the exhaust gases of a combustion engine (1) characterized in that it comprises, from upstream to downstream: • a DOC oxidation upstream oxidation member (2) or an organ NOx trap LNT; A mouth of a means (6) for introducing a reducing agent or precursor of a reducing agent for the selective catalytic reduction of the nitrogen oxides SCR; SCR selective catalytic reduction means of the nitrogen oxides, said means comprising: a selective catalytic reduction member SCR of the nitrogen oxides NOX (3) dedicated, and / or a catalytic selective reduction coating SCRF of the oxides NOX nitrogen on a particulate filter member (4); • the particulate filter member (4); • a DOCNOx downstream oxidation catalyst organ with NOx storage function (5). 2. post-treatment device according to one of the preceding claims, characterized in that the upstream oxidation catalyst member (2) comprises a nitrogen adsorbing material PNA. 3. post-treatment device according to one of the preceding claims, characterized in that the NOx storage function DOCNOx downstream oxidation catalyst member (5) comprises a metal or oxidizing metals (s) chosen from Pt and / or Pd and NOx storage materials in the form of nitrates and / or nitrites. 4. post-treatment device according to one of the preceding claims, characterized in that the DOCnox downstream oxidation catalyst member NOx storage function (5) comprises / (s) storage material (s) NOx under form of nitrates and / or nitrites from elements with basic properties, especially in the form of oxide (s) or metal aluminate (s) such as Ba barium, Neodymium Nd, lanthanum La or Praseodymium Pr, or in the form of mixed metal oxides such as solid solutions of Ce-Zr-Pr, Zr-Ce-La, Zr-Pr, Ce-Zr-La-Pr, Al-Ba, Al-Ba-Ce or Al-La-Ce. 5. post-treatment device according to one of the preceding claims, characterized in that the NOx storage function DOCNox downstream oxidizing organ (5) is free of rhodium. 6. post-treatment device according to one of the preceding claims, characterized in that the selective catalytic reduction means SCR nitrogen oxides, comprise: - a selective catalytic reduction member SCR nitrogen oxides NOx (3 ), and - a selective catalytic reduction coating SCRF NOx nitrogen oxides on a particulate filter member (4), and in that said catalyst member for selective catalytic reduction of nitrogen oxides (6) is a length at least twice as small (L1), in particular at least 2.2 to 3 times smaller than the length (L2) of the particulate filter (7). 7. Exhaust line of a heat engine, characterized in that it integrates the after-treatment device according to one of the preceding claims. 8. A motor vehicle delimiting a space under the hood and a space under body, and comprising a heat engine connected to the exhaust line according to the preceding claim, characterized in that the engine and the aftertreatment device of the exhaust line are arranged in the space under hood. 9. A motor vehicle delimiting a space under the hood and a space under body, and comprising a heat engine connected to the exhaust line according to claim 7, characterized in that: - the engine, the upstream oxidation catalyst member DOC (2) or a NOx LNT trap member, the mouth of an introducing means (6) for reducing or reducing the precursor of a reducing agent for the selective catalytic reduction of nitrogen oxides SCR, the catalytic reduction means selective SCR of the nitrogen oxides and the particulate filter member (4) are arranged in the undercap space; and in that the DOCNOx downstream oxidation catalyst member with NOx storage function (5) is arranged in the underbody space. 10. A method of implementation of the after-treatment device according to one of claims 1 to 6, characterized in that: - the gearbox or the gear reducer precursor is under-injected by the mouth of the introduction means. (6) reductant or precursor of a reducing agent for the selective catalytic reduction of nitrogen oxides SCR, so as to saturate NOx DOCnox downstream oxidation catalyst member NOx storage function (5), - the reductant or the reducing precursor is then over-injected to reduce the maximum NOx by reduction, the excess NH3 at the outlet of the particulate filter then being treated by oxidation with the DOCNOx downstream oxidation catalyst element with storage function NOx (5) as long as said organ contains stored NOx.